1. Field of the Invention
The present invention relates to a light transmitting glass fiber used in a optical telecommunication. In particular, it relates to a thin layer-coated light transmitting glass fiber (an outer diameter of an coating: at most 200 .mu.m). In the present invention, a coating resin of a resin-coated optical fiber is improved.
2. Description of Related Art
Because a light transmitting glass fiber (optical glass fiber) has a poor mechanical property and a poor light transmitting property in a state of a drawn fiber, it has a coating layer such as a polymeric material on its circumference.
As the polymeric material, an ultraviolet curing resin (hereinafter referred to as "UV resin") is generally used in view of the productivity. The .coating generally has two-layer structure. An inner layer is a relatively soft buffer layer (the coating outer diameter: about 200 .mu.m, Young's modulus: about 0.1-0.2 kg/mm.sup.2) and an outer layer is a rigid protecting layer (the coating outer diameter: about 250 .mu.m, Young's modulus: 30-100 kg/mm.sup.2).
Recently, the higher density of a telecommunication cable is required for changing the cable to an optical fiber cable in a telephone network.
In a tape slot cable which has the gathered plural tape glass fibers formed by parallel tying plural UV resin-coated optical fibers into a bundle and coating the bundle with a common UV resin coating, the conventional coating outer diameter (about 250 .mu.m) cannot give the higher density. Accordingly, the thin coating (the outer diameter of the outermost layer: about at most 200 .mu.m) is developed.
However, the thin layer-coated optical fiber (the outer diameter of the coating layer: at most 200 .mu.m) comprising a conventional UV resin (Young's modulus: 30-100 kg/mm.sup.2) as the coating material of the outermost layer gives an extreme decrease of lateral pressure resistance because of the thin coating layer.
Thus, in order to improve the lateral pressure resistance in the thin layer-coated optical fiber (the outer diameter of the coating layer: at most 200 .mu.m), the UV resin of the outermost layer is intended to have a very high Young's modulus, whereby giving substantially the same lateral pressure resistance as the conventional coated optical fiber (the outer diameter of the coating layer: about 250 .mu.m)
A term "lateral pressure property" herein means that a fine curvature arises to increase the light transmittance loss of the fiber when an external pressure (lateral pressure) is applied to the fiber. The term "lateral pressure resistance" means that the increase of the light transmittance loss is inhibited on the application of the lateral pressure.
In the thin layer-coated optical fiber (the outer diameter of the coating: at most 200 .mu.m) which has an outermost coating layer of a UV resin having a very high Young's modulus of at least 100 kg/mm.sup.2, the light transmitting property is deteriorated and the screening break strength decreases.
The reason therefor seems to be that since the UV resin used for the outermost layer has a high Young's modulus and a very low elongation (about 6%),
(1) a scratch on the coating surface easily becomes a crack and the outermost layer buckles and gives the ununiform distortion to the glass part so as to deteriorate the light transmitting property, and PA1 (2) the fiber easily breaks at the screening because of a growth of the crack. PA1 The term "screening" herein means a test which ensures a minimum strength in longitudinal direction by applying a constant pulling force to a whole length of the fiber.
The term "elongation" herein means a ratio in percentage of an elongated length to an original length until a sample breaks in a tensile test.